This invention relates to the field of wind turbines and specifically to a family of airfoils for use the blade of such a turbine.
In the past, conventional aircraft airfoil designs have been used in wind turbines. Such aircraft airfoil designs have created problems with the turbines. For clean blade conditions, these airfoils result in excessive power in high winds (which leads to burned-out generators). To avoid this problem, blades have been pitched more toward stall to reduce peak power which results in poor rotor power-to-thrust ratios and high wind-farm array losses. To overcome excessive peak power, the instant invention utilizes a tip region, outboard region and root region airfoil designed to restrain or reduce the maximum lift coefficient.
Another problem with such conventional aircraft airfoils, which has been specifically addressed by the instant invention, occurs when the blades produce inadequate energy output due to the blades becoming soiled with insect accumulation and airborne pollutants. The soiling of the blades creates airfoil roughness which affects the airfoil maximum lift coefficient and the desired output. In aircraft, such roughness is not of major concern since aircraft typically fly in clean air at high altitudes and have scheduled cleanings, but in wind turbines such cleaning is expensive and often difficult.
In the past, airfoils for wind turbine blades have been very susceptible to the effects of roughness. The instant invention is concerned width an improved family of airfoils for the blade of a horizontal-axis wind turbine, wherein the airfoil's maximum lift coefficient is designed to be largely insensitive to leading edge roughness.
An airfoil for the rotor blades of wind turbines is disclosed in U.S. Pat. No. 4,732,542 to Hahn et al. The patent, however, is directed to particular construction techniques, and not to an aeronautical design to minimize roughness of effects.
Two airfoils for turboprop aircraft are disclosed in U.S. Pat. No. 4,941,803 and 4,830,574 to Wainauski et al. Notably, the patents contemplate the use of the blades in a prop fan but not in a wind turbine. The airfoils of the patents are generally thinner with a thickness range of only 2% to 20% along the blade, whereas the range for the instant invention is 16%-24%. The airfoils of the patents were designed for high Math numbers and high blade solidity. Also, the maximum lift coefficients for the Wainauski et al. patents were not designed to be insensitive to roughness effects. Both patents, also, discuss foreign body damage, but not roughness effects. Thus they are more concerned with damage of the leading edge rather than the accumulation of insects and airborne contaminants. Again, cleaning processes associated with aircraft and operation in clean air minimize roughness concerns, whereas such cleaning is expensive and surface contamination prior to cleaning affect total energy output, for wind turbines.
U.S. Pat. No. 4,606,519 to Fertis et al. is directed to an airfoil having an offset in the upper surface. The airfoil is so designed to improve the lift and stall characteristics of the wing at high, cruising airspeeds, without substantially increasing the drag. Minimizing roughness sensitivity is not a concern of the patent. The instant invention, in contrast, contemplates a smooth suction surface without the discrete step or offset feature of the patent.
Roughness effects are also not discussed in U.S. Pat. No. 4,459,083 to Bingham. This patent is specifically directed to airfoils for rotor type aircraft and as such is directed to different concerns. The airfoil of this patent is specifically designed to reduce compressibility effects and provide a high drag divergence Mach number. The airfoils of the instant invention, in contrast, were not designed to have either of these features.
Laminar flow over an airfoil is the concern of U.S. Pat. No. 4,619,423 to Holmes et al. Specifically, the patent is concerned with shaping the interface mechanism at the juncture of the leading edge structure with both the upper and lower main wing skin panels, to allow laminar flow to occur over both the upper and lower surfaces of the airfoil. Thus the patent uses passive geometric interface devices to help maintain laminar flow over a discontinuity in the airfoil surface. However, maintaining laminar flow by shaping an interface mechanism is not a concern nor a design requirement for the airfoils of the instant invention.
The airfoil of U.S. Pat. No. 4,830,315 to Presz, Jr., has a relatively thin trailing edge with a series of troughs and ridges to form wave-like surface undulations. Again, the patent to Presy, Jr. et al. is not concerned with roughness created by insects and airborne contaminants. It is further noted that roughness is generally more pronounced on the leading edge of the airfoil, whereas the patent is concerned with trailing edge geometry to delay boundary layer separation without creating a noticeable drag penalty.
A non-smooth surface shape is disclosed in U.S. Pat. No. 5,114,099 to Gao. In contrast, the surface of the instant invention is of the conventional, generally smooth, type. The surface shape of the patent attempts to reduce drag. Roughness and its effects are not the concern of the patent.
In summary, none of the above cited patents specifically deal with an airfoil design to minimize roughness sensitivity, except for U.S. Pat. No. 4,619,423. That patent, however, is concerned with an interface mechanism to help maintain laminar flow over the airfoil surface.